1. Field of the Invention
This invention relates to volatile gas detectors and particularly to a portable photoionization detector (PID).
2. Description of the Related Art
Photo-ionization detectors (PID) can detect volatile organic gases or compounds. A conventional portable PID 10 is illustrated in FIG. 1. PID 10 includes an ultraviolet (UV) lamp 12, which produces high energy photons having an energy above 9.2 electron volts (eV). The high energy photons from UV lamp 12 are directed into an ionization chamber 14 through an optical window 16. The some of the photons collide with molecules of volatile gases having ionization potentials below the energy of the photons. Such collision ionizes the molecules, creating detectable ions and electrons.
PID 10 additionally includes an ion detector 18 having a pair of electrodes 20 and 22. Ion detector 18, typically made of a metal, has a high voltage (e.g., greater than 150 V) applied across electrodes 20 and 22 to generate an electrical field. Accordingly, first electrode 20 is electrically biased to attract positively charged particles and second electrode 22 is biased to attract negatively charged particles. Second electrode 22 repels the ions towards first electrode 20 which is simultaneously collecting the volatile gas ions. As a result, a current is produced with which the concentration of the volatile gas can be measured. The magnitude of this measurement current depends on the number of ions produced and therefore on the concentration of ionizable molecules and the intensity of the UV light in ionization chamber 14. If the UV light intensity is constant, the measurement current can be converted to the concentration, in part per million (ppm), of the volatile organic compounds.
In PID 10, there is a space 24 between optical window 16 and second electrode 22. Space 24 is a "dead zone," in which positive ions are trapped. The positive polarity of second electrode 22 prevents positive ions in space 24 from reaching first electrode 20. Accordingly, the configuration of electrodes 20 and 22 with dead space 24 inhibits the production and collection of ions and can reduce the sensitivity or accuracy of PID 10. For example, current PID devices typically can measure concentrations up to about 2,000 parts per million (ppm) of ionizable gases.
As mentioned above, the measurement current can be converted to yield the concentration of the volatile gases if the UV intensity from lamp 12 remains constant. However, UV intensity typically diminishes during the normal operation of PID 10 due to a variety of factors, including degradation of lamp 12, contamination of optical window 16, and introduction of interfering substances such as methane, carbon monoxide, or water which block or absorb UV photons in ionization chamber 14. A UV monitor 26, which is a biased electrode, is disposed in ionization chamber 14 to measure the intensity of the UV light. The UV light by striking UV monitor 26 releases electrons to produce a monitor current indicative of the intensity of the UV light. The monitor current can be used to correct for UV intensity variations when calculating the volatile gas concentration from the measurement current. The monitor current can also be used to adjust the intensity of UV lamp 12, for example, by increasing the supply voltage to lamp 12 when the monitor current indicates a low UV intensity. The monitor current, however, inaccurately measures the intensity of UV lamp 12 in the presence of ionized volatile gases. Biased monitor electrode 26 collects positive ions. As a result, the monitor current increases in the presence of ionizable gases, resulting in a less than accurate measurement of the UV intensity. Accordingly, a more accurate UV monitor is needed.
As discussed-above, contamination of PID 10, including optical window 16, reduces the UV intensity. The contamination is often a polymer-like coating caused by the deposition of metal atoms, oil film, or dust particles, during the normal use of PID 10. A user must often disassemble PID 10 to clean optical window 16. This cleaning is time consuming and burdensome. Accordingly, it is advantageous to provide a self-cleaning PID system.